Assange, 48, faces 18 counts in the US including conspiring to hack government computers and violating an espionage law. He could spend decades in prison if convicted.

Assange appeared in court dressed in a navy suit and light blue jumper, and raised his fist to supporters in the public gallery on Monday.

He was cleanly shaven, in contrast to the long beard he had grown while holed up in Ecuador's embassy.

Australian-born Assange made international headlines in early 2010 when WikiLeaks published a classified US military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.

Admirers have hailed Assange as a hero for exposing what they describe as abuse of power by modern states and for championing free speech.

His detractors have painted him as a dangerous figure complicit in Russian efforts to undermine the West and US security, and dispute that he is a journalist.

WikiLeaks angered Washington by publishing hundreds of thousands of secret US diplomatic cables that laid bare critical US appraisals of world leaders, from Russian President Vladimir Putin to members of the Saudi royal family.

In 2012, he took refuge in Ecuador's London embassy to avoid extradition to Sweden where he was accused of sex crimes which he denied, saying he believed he would ultimately be sent on to the United Sates.

He was dragged from the embassy in April after seven years and given a 50-week jail term for skipping bail. That sentence was completed but he remains in prison while his extradition case continues.

Australian Associated Press